急啊!!!请各位高手帮帮忙!

[J_X_M]

/// <summary>
/// 通过用户名得到角色Id
/// </summary>
/// <param name="user">UserInfo</param>
/// <returns>int</returns>
public static int GetUserIdByUserLoginId(UserInfo user)
{
string sql = "SELECT Id FROM UserInfo WHERE LoginId=@LoginId ";
SqlParameter[] para = new SqlParameter[]
{
new SqlParameter("@LoginId",user.LoginId)
};
return Convert.ToInt32(DBHelper.ExecuteScalar(CommandType.Text, sql, para));
}




//ExecuteScalar()

#region 执行一个命令并返回第一列
/// <summary>
/// 用指定的数据库连接字符串执行一个命令并返回一个数据集的第一列
/// </summary>
///<param name="ConnectionString">一个有效的连接字符串</param>
/// <param name="commandType">命令类型(存储过程, 文本, 等等)</param>
/// <param name="commandText">存储过程名称或者sql命令语句</param>
/// <param name="commandParameters">执行命令所用参数的集合</param>
/// <returns>用 Convert.To{Type}把类型转换为想要的 </returns>
public static object ExecuteScalar(CommandType commandType, string commandText, params SqlParameter[] commandParameters)
{
using (SqlConnection connection = new SqlConnection(ConnectionString))
{
// connection.Open();
SqlCommand command = new SqlCommand(commandText, connection);
//command.Parameters.AddRange(commandParameters);
PrepareCommand(command, connection, null, commandType, commandText, commandParameters);
object val = command.ExecuteScalar();

为什么这个val的值是空的
请各位大高手帮我看看这是哪里出了问题!
command.Parameters.Clear();
return val;
}
}
#endregion

#region 准备执行一个命令
/// <summary>
/// 准备执行一个命令
/// </summary>
/// <param name="cmd">sql命令</param>
/// <param name="conn">Sql连接</param>
/// <param name="trans">Sql事务</param>
/// <param name="cmdType">命令类型例如 存储过程或者文本</param>
/// <param name="cmdText">命令文本</param>
/// <param name="cmdParms">执行命令的参数</param>
private static void PrepareCommand(SqlCommand command, SqlConnection connection, SqlTransaction transaction, CommandType commandType, string commandText, SqlParameter[] commandParameters)
{
if (connection.State != ConnectionState.Open)
connection.Open();

command.Connection = connection;
command.CommandText = commandText;

if (transaction != null)
command.Transaction = transaction;

command.CommandType = commandType;

if (commandParameters != null)
command.Parameters.AddRange(commandParameters);
}
#endregion